Added an environment variable hack to cause the buildout to use a
local empty index when running tests. This allows most of the tests to
be run when offline and doesn't slam pypi at other times.


git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@69989 62d5b8a3-27da-0310-9561-8e5933582275